Onion (Allium cepa L.) is a member of Alliaceae family and is of immense benefit due to its dietic and medicinal values. Onion is one of the most important vegetable crops in Nigeria, where it is exported and is an important condiment in the preparation of curry and spicy dishes. Yet the yield obtained by the farmers is low 15 t ha-1 compared to Niger Republic 36 t ha-1, South Africa 26 t ha-1 Morocco 24.9 t ha-1 and Egypt 30.5 t ha-1 (FAOSTAT Data, 2005). Onion cultivation in Nigeria is confined to the semi-arid northern guinea and Sudan Savannah zones and the soils in this area are mostly low in nutrients, due to its low organic matter content which lowers the yield (Amans et al., 1996).
Inadequate N has been reported as major constraints to increased onion production. N is essential to growth and yield of onion but excessively high doses cause delay in bulb maturity and encourages bolting, which is an undesirable characteristic. Application of 80-90 kg ha-1 of N has been reported to be adequate for onion production (Umar et al., 2003; Halvorson et al., 2002). Nitrogen nutrition can influence onion bulb development, flavour and bulb quality (Randle, 2000). Samaila (2000) at Zaria, northern Nigeria and Al-Moshileh (2001) in Saudi Arabia reported a significant increase in the number of leaves per plant, plant height and yield as a result of N application on onion. However, responses up to 100-120 kg N ha-1 have been reported by Islam et al. (1999) and Singh et al. (2000).
Various research findings revealed that population density greatly influenced growth and yield of onion. Kumar et al. (1998) obtained the highest yield with spacing of 20x10 cm. Khan et al. (2002) reported a significant effect of all the growth and yield components of onion. They maintained that larger percentage of small and medium bulbs were obtained in the narrowest spacing.
Keeping in view these aspects, the present research work was initiated to determine the optimum dose of nitrogen fertilizer and appropriate intra-row spacing on the growth and bulb yield of onion under Sokoto semi-arid condition.
MATERIALS AND METHODS
Field experiments were conducted at the Usmanu Danfodiyo University Sokoto Fadama Teaching and Research farm in the 2004 and 2005 dry seasons. The treatments consisted of factorial combination of four nitrogen levels (0, 50, 100 and 150 kg N ha-1) and four intra-row spacing (10, 15, 20 and 25 cm) laid out in a split-plot design replicated thee times. Nitrogen was allocated to the main plots while intra-row spacing to the sub-plots keeping the net plot size at 4.8 m2 and a row-to-row (inter-row) distance was maintained at 20 cm.
The soils at the experimental site were sandy loam in texture with low organic matter, total nitrogen and phosphorus content. The soil reaction was moderately acidic in nature (Table 1).
The experimental area was thoroughly prepared by ploughing and harrowing. Onion variety Violet de Galmi, was raised in the nursery using the recommended management practices. Seedlings were transplanted to the field at about four weeks after sowing. N fertilizer was split-applied using urea (46% N) as prescribed by the treatments. All recommended cultural practices were adopted uniformly according to onion requirements. Data was recorded on plant height, number of leaves per plant, crop growth rate, bulb diameter, individual bulb diameter, individual bulb weight and total bulb yield.
The data collected was subjected to Analysis of Variance technique (ANOVA) using SAS (2003) computer package. Duncans Multiple Range Tests was adopted for the means comparison among treatments showing significant difference.
|| Physico-chemical properties of soils at the experimental
site during 2004 and 2005 dry seasons at Sokoto
RESULTS AND DISCUSSION
Nitrogen had significant effect on plant height of onion with the highest
value 55.85 cm obtained from application of 150 kg N ha-1 which was
at par with 55.67 cm obtained from the application of 100 kg N ha-1
(Table 2). This clearly showed that nitrogen is mainly concerned
with the vegetative growth of the plants. Similar findings were reported by
Kumar et al. (1998), Amans et al. (1996), Arboleya and Garcia
(1993), Samaila (2000) and Khan et al. (2002).
Significantly highest plant height 52.51 cm was obtained from intra-row spacing of 20 cm followed by 51.78 cm obtained when the spacing of 25 cm was used. The shortest plants 47.37 cm were observed from 15 cm intra-row spacing (Table 3). Closer spacing resulted in competition for nutrient and light thus resulting in plants that were short while the wider spaced plants had adequate space for their growth and development.
There was a significant effect of interaction between nitrogen and intra-row spacing on height of onion. At 0 kg N ha-1 20 and 25 cm intra-row spacing were at par. Similarly at 50 and 100 kg N ha-1 20 and 25 cm spacing were at par. While at 150 kg N ha-1, 10 and 15 cm intra-row spacing recorded similar height but significantly higher than 20 and 25 cm. This could be due to luxury consumption where enough nutrients were available thus preventing competition between plants. Highest plant height was obtained from 100 kg N ha-1 combined with 20 and 25 cm intra-row spacing (Table 4).
Number of Leaves per Plant
Nitrogen levels significantly affected the number of leaves per plant of
onion with the highest value (12.63) obtained from 150 kg N ha-1
but not statistically different from 12.52 obtained from 100 kg N ha-1
(Table 1). This also shows that, nitrogen played important
role in leaf production via its role in vegetative growth. Effect of nitrogen
in increasing number of leaves of onion was reported by Kumar et al.
(1998) and Khan et al. (2002) who reported that number of leaves per
plant increased with increasing nitrogen levels up to 150 kg ha-1.
|| Effect of nitrogen on the growth and yield of onion
|Within a parameter, means in a row followed by the same letter
(s) are not significantly different at 5% level using DMRT
|| Effect of intra-raw spacing on growth and yield of onion
|Within a parameter, means in a row followed by the same letter(s)
are not significantly different at 5% level using DMRT
|| Effect of interaction between nitrogen and intra-row spacing
on the growth and yield of onion
|Means in a column followed by the same letter(s) are not significantly
different at 5% level using DMRT
There was significant effect of intra-row spacing on number of leaves per
plant. Significantly higher number of leaves 11.18 was recorded with 20 and
25 cm intra-row spacing then 10, 15 cm significant effect of interaction between
N and intra-row spacing with higher values obtained at 20 and 25 cm intra-row
spacing at all levels of N. However, both 100 and 150 kg N ha-1 recorded
similar leaf number with 20 and 25 cm intra-row spacing (Table
Crop Growth Rate (CGR)
Crop growth rate was significantly influenced by the nitrogen application
up to 150 kg ha-1. It was observed that the nitrogen triggers the
growth rate of onion with the highest value 1.03 g m-2 day-1
obtained from 150 kg N ha-1. This was followed by 0.92 g m-2
day-1 obtained from 100 kg N ha-1. Control plot recorded
the lowest crop growth rate of 0.5 g m-2 day-1 (Table
2). Similarly, intra-row spacing had significant effect on CGR with highest
value at 20-25 cm spacing (Table 3). Interaction between N
and spacing was significant with 100 kg N ha-1 and 20-25 cm spacing
recording higher CGR than other N levels and spacing (Table 4).
Cured Bulb Yield
Bulb yield of onion was significantly affected by the application of nitrogen
(Table 2). Increase in N dose up to 100 kg ha-1
resulted in the increased yield of onion bulbs 40.59 t ha-1. But
further increase to 150 kg N ha-1 did not significantly increase
the yield. The lowest bulb yield of 10.29 t ha-1 was recorded from
the control plot, where no N was applied. Similar results were reported by Shaikh
et al. (1987), Patel and Patel (1990) and Khan et al. (2002).
Intra-row spacing of (15 cm) produced the highest bulb yield (31.7 t ha-1) followed by 30.81 t ha-1 recorded when 10 cm intra-row spacing was used. Widest spacing of 25 cm recorded the lowest yield 26.93 t ha-1 (Table 3).
Similarly, interaction of nitrogen and intra-row spacing had significant effect on the cured bulb yield of onion (Table 4). In contrast to growth parameters, significantly higher cured bulb yield was recorded with 15 cm intra-row spacing than other spacing at all levels of N including 0 (control). However, yield recorded with 100 and 150 kg N ha-1 and 15 cm spacing was similar and significantly higher than 0 (control) and 50 Kg N ha-1 (Table 4). 100 kg N ha-1 appeared to be the optimum for onion production with a combination of 15 cm intra-row spacing. Wider spacing reduced yield due to total reduction in the plants per hectare i.e. space is not fully utilized while close spacing of 10 cm resulted in competition for nutrients, water and light resulting in lower yield.
Nitrogen significantly affected the bulb diameter of onion with highest
8.7 cm recorded from application of 100 kg N ha-1 (Table
2). The minimum value for bulb diameter 3.75 cm was recorded from the control
plots where no nitrogen was applied. Kumar et al. (1998) and Khan et
al. (2002) also reported that bulb diameter was significantly affected by
the application of nitrogen.
The highest bulb diameter (8.12 cm) was recorded with 25 cm spacing, which was statistically at par with that recorded for 20 cm spacing (8.11 cm). The lowest bulb diameter (4.68 cm) was obtained with 10 cm spacing.
Increased nitrogen application up to 100 kg ha-1 resulted into
an increase in the bulb weight of onion. But further increase to 150 kg ha-1
did not result in the weight increase (Table 2). 150 kg N
ha-1 produced the heaviest bulbs 230.32 g which was not statistically
different from the 228.30 g obtained from the application of 100 kg N ha-1.
Therefore 100 kg N ha-1 is the most optimum technical dose for onion
bulb production. Control plots produced the lowest values for bulb weight 89.61
g due to the absence of the nitrogen, which is an important element needed for
proper growth and development of every plant including onion. Kashi and Frodi
(1998) Greenwood et al. (2001) and Khan et al. (2002) reported
significant increase in the bulb weight of onion due to increased nitrogen application.
Intra-row spacing had significant effect on the bulb weight of onion with 20 and 25 cm spacing recording similar bulb weight but significantly higher than 10 and 15 cm spacing (Table 3).
There was significant effect of interaction between Intra-row spacing and N (Table 4). Significantly higher bulb weight was recorded with 20 and 25 cm intra-row spacing at 0, 50 and 150 kg N ha-1. At 100 kg N ha-1, all spacing recorded similar bulb weight. However, higher bulb weight was recorded with 150 kg N ha-1 with 20 and 25 cm intra-row spacing than any other treatment combinations.
From this study, it can therefore be concluded that higher total tuber yield (irrespective of the bulb size) could be obtained by applying 100 kg N ha-1 in plants spaced at 15 cm intra-row spacing and 20 cm inter-row spacing. However, for large bulb size, application of 150 kg N ha-1 in plants spaced at 25 cm (25 cm spacing may be preferred over 20 cm for convenience in planting and other cultural operations) intra-row spacing and 20 cm inter-row spacing may recommended.